1. Field of the Invention
This invention relates in general to photogenerated polycarbodiimides obtained from poly(tetrazole-5-thiones). In another aspect, this invention is directed to the photogeneration of polycarbodiimides and their use in coatings and photoresists. In a further aspect the invention relates to novel, polymerizable tetrazole-5-thione, the polymers prepared therefrom, and certain multifunctional tetrazole-5-thiones. In a still further aspect, the invention is directed to photogenerated polycarbodiimides which are useful as crosslinking agents for polymers containing carboxylic acid groups.
2. Description of the Related Art
The trend in electronics is toward even higher resolution for imagining techniques, both in integrated circuits and in printed circuit boards. At the same time processing technologies (plasma treatments, solder contacts) require higher thermal stability. One approach to thermal stability is through cross-linking of the resin used in the imaging step. In general this requires a radiation treatment after development of the image or the use of a negative (crosslinking) resists. Negatives have had the disadvantage of requiring solvent development of the uncrosslinked portion of the resist film. However, solvents generally swell the crosslinked portion resulting in a loss of resolution. Moreover, the use of organic solvents creates undesirable environmental problems. Accordingly, an aqueous developable negative resist would avoid the problem of swelling and reduce solvent emissions as well.
Photoresists are photosensitive materials which change their solubility after exposure to light. They are typically novolac resins which have DANQ (diazonaphthoquinone) sensitizers attached to the polymer backbone or added to the resist formulation. DANQ sensitizers act as inhibitors to decrease the solubility of the photoresist in basic aqueous solutions. Irradiation of films through a mask causes the formation of indenecarboxylic acid photo-products which renders the novolac resist (in the exposed areas) soluble in aqueous base. During development, preferential dissolution of the photoresist in the irradiated areas results in a positive image.
Polycarbodiimides are excellent crosslinkers for a variety of functional polymers containing carboxylic acid or hydroxyl moieties. They are particularly effective at low temperatures and accordingly are useful for crosslinking carboxylic acid-containing polymers.
Prior to the present invention, it was known that if carbodiimides are added to photoresist formulations, negative images can obtained. When, for example, N,N'-dicyclohexyl carbodiimide is added to a novolac photoresist, it reacts reversibly with novolac hydroxyls to produce isoureas. Isoureas are "blocked" reactive agents which deactivate indenecarboxylic acid photo-products in the irradiated areas during postbake. Since the carboxylic acid groups are deactivated in the irradiated areas, the solubility in these areas are greatly reduced. After postbake, the resist is flood exposed to convert the remaining photoactive compound to indenecarboxylic acid groups; development then gives a negative image of the mask. Since this chemistry effects the solubility of the resist and does not appear to crosslink the resist in the irradiated areas, no increase in the thermal stability of the images is observed.
Since the presence of carbodiimides were shown to be useful in photoresist formulations, a study was undertaken to determine if carbodiimides could be generated in situ. Accordingly, tetrazole-5-thiones and polymers containing pendant tetrazole-5-thione groups were prepared and evaluated for their effectiveness as a source from which carbodiimides, both in monomeric and polymeric form, could be photogenerated.